126 • Marine Minerals: Exploring Our New Ocean Frontier 



Figure 4-4.— Multi-Beam Bathymetry Products 



23-50' 



a) Contour map of part of tfie Kane Fracture Zone, Mid- 

 Atlantic Ridge 



b) Three-dimensional Mesh Surface Presentation of the same 

 data. Charts and 3-D presentations such as these are impor- 

 tant tools for geological and engineering investigations of 

 the seafloor. 



SOURCE: R. Tyce, Sea Beam Users Group. 



Improvements in seafloor mapping have resulted 

 from the development of multi-beam bathymetry 

 systems (table 4-4), the application of heave-roll- 

 pitch sensors to correct for ship motion, the im- 

 proved accuracy of satellite positioning systems, and 

 improved computer and plotter capability for proc- 

 essing map data.^^ These improvements make 

 possible: 



"C. Andreasen, "Nationa] Oceanic and Atmospheric Administra- 

 tion Exclusive Economic Zone Mapping Project," in Proceedings: 

 The Exclusive Economic Zone Symposium: Exploring the New Ocean 

 Frontier, M. Lockwood and G. Hill (eds.), conference sponsored by 

 National Oceanic and Atmospheric Administration, U.S. Department 

 of the Interior, Smithsonian Institution, and Marine Society, held at 

 Smithsonian Institution, Oct. 2-3, 1985, pp. 63-67. 



1 . much higher resolution for detecting fine scale 

 bottom features; 



2. a significant decrease in time required for 

 making area surveys; 



3. nearly instantaneous automated contour 

 charts, eliminating the need for conventional 

 cartography;^* and 



4. the availability of data in digital format. 



Deep-Water Systems 



Swath bathymetric systems are of two types: 

 those designed to operate in deep water and those 

 designed primarily for shallow water. The principal 

 deep-water multi-beam systems currently in use in 

 the United States are Sea Beam and SASS. Sea 

 Beam technology, installed on NOAA's NOS ships 

 to survey EEZ waters deeper than 600 meters, first 

 became available from General Instrument (GI) 

 Corp. in 1977. GI's original multi-beam bathymet- 

 ric sonar, the Sonar Array Sounding System (or 

 SASS) was developed for the U.S. Navy and is not 

 available for civilian use. Sea Beam is a spinoff from 

 the original SASS technology. 



Sea Beam is a hull-mounted system, which uses 

 16 adjacent beams, 8 port and 8 starboard, to sur- 

 vey a wide swath of the ocean bottom on both sides 

 of the ship's track (figure 4-5). Each beam covers 

 an angular area 2.67° square. The swath angle is 

 the sum of the individual beam width angles, or 

 42.67°. With the swath angle set, the swath width 

 depends on the ocean depth. At the continental shelf 

 edge, i.e., 200 meters, the swath width is about 150 

 meters at the bottom; in 5,000 meters (16,400 feet) 

 of water, the swath width is approximately 4,000 

 meters. Therefore, Sea Beam's survey rate is 

 greater in deeper waters. By carefully spacing ship 

 tracks, complete (or overlapping) coverage of an 

 area can be obtained. The contour interval of 

 bathymetric charts produced from Sea Beam can 

 be set as fine as 2 meters. 



The Navy's older SASS model uses as many as 

 60 beams, providing higher resolution than Sea 

 Beam in the direction perpendicular to the ship's 

 track (Sea Beam resolution is better parallel to the 

 ship's track). In current SASS models, the outer 

 10 or so beams are often unreliable and not used.^' 



"H.K. Farr, "Multibeam Bathymetric Sonar: Sea Beam and 

 Hydrochart," Marine Geodosy, vol. 4, No. 2, pp. 88-89. 

 ^'Vogt and Tuchoike, "Imaging the Ocean Floor," p. 37. 



